1. Field of the Invention
The present invention relates to piezoelectric resonators and electronic devices incorporating such piezoelectric resonators. More particularly, the present invention relates to piezoelectric resonators including piezoelectric layers such as piezoelectric films or piezoelectric substrates and electrodes disposed on both main surfaces of the piezoelectric layers. In addition, the invention relates to electronic devices incorporating such piezoelectric resonators, such as filters.
2. Description of the Related Art
As a background relating to the present invention, conventional piezoelectric resonators are described in Japanese Examined Patent Application Publication No. 46-25579, Japanese Unexamined Patent Application Publication No. 8-154032, and Japanese Unexamined Patent Application Publication No. 5-160666.
Japanese Examined Patent Application Publication No. 46-25579 describes a piezoelectric oscillator including a multi-layer substrate, first and second electrodes, and a piezoelectric film.
Japanese Unexamined Patent Application Publication No. 8-154032 describes a piezoelectric resonator including a piezoelectric substrate, first and second electrodes disposed on one main surface of the piezoelectric substrate, a third electrode disposed on the other main surface of the piezoelectric substrate. In this piezoelectric resonator, two vibrating sections are connected in series at locations where the first and second electrodes oppose the third electrode.
In addition, Japanese Unexamined Patent Application Publication No. 5-160666 describes a piezoelectric resonator including a piezoelectric substrate and electrodes disposed on both main surfaces of the piezoelectric substrate.
Concerning the piezoelectric oscillator described in Japanese Examined Patent Application Publication No. 46-25579, in order to reduce a resonance resistance, the dimensions of the electrodes disposed on both main surfaces of the piezoelectric film must be expanded to increase the area in which the electrodes oppose each other so that a greater capacitance can be obtained. On the other hand, expanding the dimensions of the electrodes on the main surfaces of the piezoelectric film causes unnecessary vibrations, which thereby leads to the deterioration of resonant characteristics.
Thus, in order to prevent the resonant characteristics from being deteriorated while reducing the resonance resistance and neglecting the unnecessary vibrations, one possible method is to connect in parallel a plurality of piezoelectric resonators having small electrode-opposing areas. However, in this case, since the entire chip size increases, there is a problem in that wiring for connecting piezoelectric resonators is elongated.
Furthermore, in the piezoelectric resonator described in Japanese Unexamined Patent Application Publication No. 8-154032, when the resonant resistance of each vibrating section is represented by the symbol R, the entire resonant resistance is equivalent to 2R. In order to reduce the entire resonant resistance, it is necessary to reduce the resonant resistance of each vibrating section. In order to do so, the electrode-opposing area needs to be increased. As a result, unnecessary vibrations are no longer negligible, resulting in the deterioration of the resonant characteristics.
Furthermore, in the piezoelectric resonator described in Japanese Unexamined Patent Application Publication No. 5-160666, when electrode patterns disposed on the main surfaces of the piezoelectric substrate deviate from each other, the electrode-opposing area varies and the resonant frequencies become different from each other. Thus, even with n pieces of the piezoelectric resonators connected in parallel, the resonant resistance is not reduced to be 1/n.
In order to overcome the problems described above, preferred embodiments of the present invention provide a very compact piezoelectric resonator having excellent resonant characteristics and very small resonance resistance.
Also, other preferred embodiments of the present invention provide an electronic device including such a novel piezoelectric resonator.
According to a first preferred embodiment of the present invention, a piezoelectric resonator which vibrates in a thickness vibration mode, includes a piezoelectric layer, a first electrode disposed on a main surface of the piezoelectric layer, the first electrode including a leading electrode and at least one vibrating electrode connected to the leading electrode, a second electrode disposed on the other main surface of the piezoelectric layer, the second electrode including a leading electrode and at least one vibrating electrode connected to the leading electrode, and a plurality of vibrating sections at which the at least one vibrating electrode of the first electrode overlaps with the at least one vibrating electrode of the second electrode via the piezoelectric layer, the plurality of vibrating sections being connected in parallel.
In this piezoelectric resonator, preferably, a distance between adjacent vibrating sections is equal to or greater than about xc2xd of a wavelength at which the adjacent vibrating sections vibrate.
According to a second preferred embodiment of the invention, an electronic device includes at least one piezoelectric resonator according to the first preferred embodiment described above.
In the piezoelectric resonator according to preferred embodiments of the present invention, in each vibrating section, the dimensions of the vibrating electrodes of the first and second electrodes can be made small enough to prevent the influence of unnecessary vibrations. Thus, the resonant characteristics are not deteriorated.
Moreover, in piezoelectric resonators of preferred embodiments of the present invention, even when the first electrode deviates from the second electrode, the area in which the vibrating electrodes oppose each other does not change. Thus, the resonant resistance of each vibrating section and the resonant frequency do not change.
In addition, in piezoelectric resonators of preferred embodiments of the present invention, the plurality of vibrating sections are preferably formed by overlapping the vibrating electrodes of the first electrode with the vibrating electrodes of the second electrode via the piezoelectric layer. Thus, the plurality of vibrating sections can be arranged in a very small area.
In addition, in piezoelectric resonators according to preferred embodiments of the present invention, since the plurality of vibrating sections is electrically connected in parallel to each other, the resonant resistance is smaller than the resonant resistance of each vibrating section.
Furthermore, in the piezoelectric resonators according to preferred embodiments of the present invention, when the distance between the adjacent vibrating sections is equal to or greater than about xc2xd of the wavelength at which the adjacent vibrating sections vibrate, vibrations leaking in lateral directions are attenuated and eliminated. Consequently, the vibrations of the adjacent vibrating sections do not interfere with each other and thereby the resonant characteristics are not deteriorated.
The above and other features, elements, characteristics and advantages of the present invention will be further clarified by detailed explanations of preferred embodiments of the present invention with reference to the drawings.